Try this place, their prices are EXCELLENT and they have exactly what you need. I should know since i'm purchasing the same thing. It's black acrylic sheeting. I asked around and i've met a few people online that have bought acrylic from this place to use for corner overflows, and overflows in general.

You can choose the thickness, color, width, length, etc.. But in your case, since your just looking for acrylic to cover that overflow, you could get really thin sheets and that's very inexpensive. It's only when you get into 1/4" thickness does it get a tad pricey.

Yeah its hard here in Aus, There is a place in Vic that people do get it from, But its a while away from my house. After the coralline algae and rocks are in the tank, you wont notice the weir too much.
So it should be ok. If it really does worry me in the future i can always put a sheet of acrylic there.